The present invention relates to methods for determining the formation parting or fracture parting or propagation pressure of an underground formation and more particularly to such a method in which fluids are injected into a wellbore. 2. Setting of the Invention
As hydrocarbons are produced from a formation, the pressure in the formation gradually drops thus diminishing the rate of production. At some point it becomes economically justifiable to adopt enhanced recovery techniques in order to produce additional oil and gas from the formation. One widely applied technique involves injection of water and/or gas into the reservoir. Wells which are no longer producing may be used to inject water via pumps into a formation or new injection wells may be drilled. As water is injected, it moves through the reservoir displacing oil to producing wells.
In the case of certain types of wells, adjacent injection wells can be used to inject a gas, such as carbon dioxide, into the formation thereby driving hydrocarbons toward the producing well.
When injecting a fluid into a well to enhance recovery of an adjacent producing well, it is desirable to maintain the injection pressure below the pressure at which the formation parts or fractures. As fluid is injected into a well, the pressure in the formation surrounding the well increases until at some point the formation parts thereby establishing or opening a pre-existent fluid flow channel. Injected fluid flows into the channel and may be dispersed in the formation in a manner which decreases the effect of the injected fluid on the producing well. Even worse, an uncontrolled fracture might communicate with the producing wellbore, thereby forming a "pipeline" directly between the injecting well and the producing well. Thus, it is desirable to inject fluid into an injection well at a rate which maintains the pressure in the formation below the parting pressure.
It should be noted that some injection wells may include a fracture which is propped open. In such a case, fluid is injected into the well to fracture the formation. Thereafter, a propping agent is placed in the well to fill the fracture and maintain it in an open condition. When such a well is used as an injection well, it is desirable to inject fluid at a rate which prevents extension of the existing fracture in order to avoid any detrimental effect of fluid injection on the adjacent producing well or wells.
As can be seen from the foregoing, it is desirable to know at what pressure a particular formation will fracture. There exists a prior art method for determining formation parting pressure. In the prior art method, fluid is injected into the wellbore using a series of constant injection rates. The injection rate is increased from low to high in a stepwise fashion with the rate at each step being maintained at a constant level. It is also assumed that a steady state or a pseudo steady-state condition is achieved during each injection step. This may require that each step is continued ranging from a few hours to several tens of hours. The injection rates and the resulting injection pressures are measured during the test. The parting pressure can be determined from a plot of pressure at the end of each injection step against the corresponding rate for that step. Ideally, the plot exhibits straight line behavior below parting pressure and a reduced slope when parting pressure is exceeded. The pressure where this change in slope occurs is identified as the formation parting pressure. It can be seen that the accuracy of this test increases with the number of steps. In order to obtain a reasonably accurate indication of formation parting pressure, a series of several steps, each of which consists of a substantially different constant injection rate and a constant but long injection time period, is considered necessary.
In running the prior art test, it may be necessary to substantially disrupt injection well operations. In other words, the injection well to be tested may be injecting at a selected rate in order to increase recovery from adjacent producing wells. If so, the well may have to be shut-in or the rate may have to be substantially lowered in order to perform the prior art test. Injection operations cannot be resumed until after the several step test is performed.
There exists a need for a method for determining formation parting pressure which can be accomplished in a shorter time than prior art methods.
Moreover, there exists a need for such a test which can be performed quickly on an injection well which is injecting fluid into the formation.
SUMMARY OF THE INVENTION
The instant invention comprises a method for determining the parting pressure of a formation having a wellbore therethrough. Fluid is injected into the wellbore at a first rate for a first period of time. The rate of injection is changed to a second rate, which causes the formation pressure to rise above the formation parting pressure during a second step. Fluid injection is maintained at the second rate for a second period of time. The pressures in the wellbore during the first and second periods are measured. The pressure data obtained is normalized and the formation parting pressure is determined by locating the point at which the normalized data for the second time period deviate from the normalized data for the first time period.